1. Field of the Invention
This invention relates in general to a land mobile-satellite communication system and, in particular, to such a communication system which utilizes vehicles such as automobiles as carriers for newly introduced mobile repeater stations.
2. Discussion of Relevant Art
Recently, several land mobile-satellite communication systems have been planned in which portable telephone terminals can communicate with each other through communication satellite stations mounted on each of a plurality of low earth satellites. In such land mobile-satellite communication systems, if both calling and called portable telephone terminals locate in the same service area provided by one communication satellite station, a communication link or speech path between both the calling and the called telephone terminals can be formed through the communication satellite station.
On the other hand, if the called telephone terminal is not located in the service area provided for the calling telephone terminal, a communication link between the calling and the called portable telephone terminals will be made through an inter-satellites communication link.
In the communication systems described above, the radio transmission paths formed between the portable telephone terminals and the low earth communication satellite stations are much shorter than that which may be formed between the portable telephone terminals and stationary satellite stations of high altitude above the equator. As a result, it is possible to form communication links of high quality between the portable telephone terminals and the low earth communication satellite stations without imposing excess antenna gain or excess transmission power on either the communication satellites or the portable telephone terminals. Such land mobile-satellite communication systems are disclosed in U.S. Pat. Nos. 5,970,414; 6,072,768; 6,097,752; 6,108,561 etc.
However, to make such a communication system more economical, it will be necessary to reduce the size of the low earth communication satellite stations, including antennas, and to reduce power consumption in both the communication satellite stations and the portable telephone terminals.
Further, to make the communication more reliable and of higher quality, it will be necessary to improve SN ratio in the communication links between the portable telephone terminals and the low earth communication satellite stations.
Moreover, it is necessary to provide various services to the subscribers of such a land mobile-satellite communication system which includes an accessibility to the conventional land mobile communication systems and the conventional land networks such as Internet.